Glycoside surfactants have been known for at least sixty years. They are nonionic surfactants with low toxicity and gentleness to the skin. They can be made from renewable resources and are rapidly degraded in the environment.
In spite of the excellent properties of the glycoside surfactants, they have not achieved great commercial acceptance because of the difficulties encountered in their production. Rohm & Haas Corporation has been providing alkyl glycosides in which the alkyl group is formed from a mixture of fatty alcohols having 8 and 10 carbon atoms. The compositions are sold under the trade names BG-10 and CG-110. The BG-10 composition is a dark, almost black material and is used only in commercial formulations. The CG-110 material is a light colored product useful for consumer applications.
Early processes for the preparation of glycoside surfactants were two-step processes. The first step comprised the reaction of a lower alcohol having 1 to 6 carbon atoms with a saccharide source in the presence of an acid catalyst to form a glycoside. The glycosides prepared from the lower alcohols did not have useful surfactant properties. Since water is soluble in the alcohols, the reaction mixture could contain a substantial quantity of water (see U.S. Pat. No. 4,721,780). The higher glycoside surfactants in which the organic group attached to the glycoside moiety contains more than about 7 carbon atoms are then prepared by transacetalization of the lower glycoside with alcohols containing at least 7 carbon atoms under substantially anhydrous conditions.
More recently, aliphatic glycosides having aliphatic groups with from 7 to 22 carbon atoms have been prepared by a "direct process". In the "direct process" a long chain fatty alcohol is reacted with a source of saccharide in the presence of an acid catalyst under conditions in which the water formed in the reaction is removed as quickly as it is formed to maintain the water content of the reaction mixture at as low a level as is reasonably possible. The water formed in the reaction is only sparingly soluble in the fatty alcohol and any undissolved water results in the rapid formation of unwanted byproducts. The parameters of the "direct process" with a less than ideal catalyst and neutralization procedure are disclosed in U.S. Pat. No. 3,839,318, which is incorporated herein by reference. Other patents such as U.S. Pat. No. 4,939,245, U.S. Pat. No. 4,950,743 and U.S. Pat. No. 5,003,057 also disclose the "direct process" and are incorporated herein by reference.
The glycoside surfactants are formed by the reaction of the alcohol having more than about 7 carbon atoms and preferably more than about 8 carbon atoms with a saccharide source under anhydrous conditions in the presence of an acid catalyst. The reaction is generally carried out in the presence of a stoichiometric excess of the alcohol and at least a sufficient amount of alcohol to maintain the reaction mixture in a fluid state. Generally, the reaction mixture contains from about 1.5 to about 10 moles of alcohol per mole of saccharide moiety.
The reaction is carried out at a temperature in the range of from about 90.degree. C. to about 145.degree. C. under a reduced pressure. The high temperature and reduced pressure provide for rapid removal of the water formed in the reaction from the reaction mixture. The temperature and reduced pressure at which the reaction is carried out is dependent upon the alcohol used and the amount of discoloration which can be tolerated in the finished product. The lower molecular weight alcohols generally react at lower temperatures and at higher pressures, since at lower pressures the lower molecular weight alcohols tend to vaporize and change the composition of the reacting mixture.
After the source of saccharide has been substantially all reacted with the alcohols or polymerized to form a polymer, the acid catalyst is neutralized.
After the catalyst has been neutralized, it is generally accepted procedure that the unreacted or excess alcohol is then separated from the reaction mixture. Generally, it is desirable to have as low a content of the higher alcohol in the product as possible. The presence of higher alcohols are known to reduce the surfactant activity of the composition and to impact the odor of the product. Generally, the amount of alcohol remaining in the product is generally less than about 5% by weight of the product and preferably less than about 2% by weight and most preferably less than about 1.0% by weight of the product.
The alcohol is generally removed from the reaction mixture by heating the reaction mixture at a reduced pressure. Preferably, the alcohol is separated from the glycoside product in a thin film evaporator such as disclosed in U.S. Pat. No. 4,223,129 or EP 077 167. U.S. Pat. No. 3,565,885 and U.S. Pat. No. 4,393,203 disclose that the most preferred method for removing the unreacted alcohol from the reaction mixture is in a wiped film evaporator. Especially with longer chain alcohols, a wiped film evaporator is particularly useful in that the glycoside product is exposed to the high temperature for only a short period of time and the degradation caused by the vaporization process is substantially reduced.
U.S. Pat. No. 5,079,350 discloses that the most preferred method for removing unreacted alcohol from a glycoside surfactant product is to contact the alkyl glycoside and alcohol mixture with a stream of inert gas under reduced pressure in a wiped film evaporator maintained at a temperature in the range of about 140.degree. C. to about 200.degree. C. The process is disclosed as substantially removing all of the unreacted alcohol and the odor from the glycoside product.
Wiped film evaporators have been recognized as being useful for removing high boiling point materials from heat sensitive products. However, wiped film evaporators are an expensive apparatus and have a limited surface area for a unit volume of the evaporator. It would be advantageous to be able to reduce the evaporation load which is required of the wiped film evaporator. The prior art teaches that the glycoside products are sensitive to heat and the color of the product deteriorates when the reaction mixture is exposed to a high temperature for a relatively long period of time. One skilled in the art faced with the problem of removing unreacted fatty alcohol from a glycoside product would select a process in which the glycoside product is exposed to an elevated temperature for as short a period of time as possible.